NikonPoint

To add to this topic I and Oskar Ojala tested the Voigtländer 20/3.5 against the Zeiss 18/3.5 on the D300 and the D700. I will briefly comment on the D700 results here.

Tests were made at f/4, f/5.6, and f/8 on a brick church (Uspenski Cathedral in Helsinki). The shooting angle was chosen so that focusing errors would be easy to detect and near and far subject areas were located at edges as well as center of the frame.

Center performance on 12 MP FX (D700) was very similar with both lenses and the aperture dependency was minimal.

At peripheral parts of the image, at f/4 and f/5.6 the Voigtländer exhibited significant internal flare at high contrast edges and was a bit softer than the Zeiss, which retained sharpness to the edges of the frame and had no internal flare. By f/8 the image quality was very close to equal.

The Voigtländer exhibited no ghosts from the side-lighting sun, but the Zeiss did produce a single slightly visible ghost at one point (f/8 visible, f/5.6 harder to see, f/4 essentially not visible).

Overall, for 12 MP FX use the Zeiss is clearly superior in terms of edge sharpness and the internal flare of the Voigtländer was a significant artifact, but if you only shoot stopped down to f/8 or so, I don't think there is any significant reason to choose the Zeiss over the smaller and cheaper Voigtländer. For wide aperture use I think the Voigtländer is best used on DX only.

DX performance was similar with both lenses except for the one ghost which was again visible with the Zeiss.

The Voigtländer gave a slightly brighter image (more exposure) overall, aperture for aperture.

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I think the 18 ZF is more contrasty and saturated than the Voigtländer in the peripheral parts of the image. In the center part (DX area) they're very similar.

I think either lens is a good choice. The Voigtländer's compactness is nice. For FX use you may need to stop the Voigtländer down to f/8. The Zeiss produces nice results edge to edge even at wider apertures though at f/4 the vignetting is prominent. At f/5.6, vignetting is very slight, and gone by f/8. With the Zeiss, contrast peaks at f/5.6 and this is the aperture I use most often along with f/8.

I think the Voigtländer is a good choice for DX users since it's so tiny, well made and less expensive. For FX use, the 18mm ZF performs better at f/4 and f/5.6 but if you're short on money or space in the bag, you may still opt for the Voigtländer since the quality difference at f/8 is minimal. In fact, the Voigtländer may have better resistance to ghosting than the Zeiss which displayed a single ghost in the test image (due to the sun). The Voigtländer has the CPU contacts so the aperture and focal length are recorded without going through the non-CPU list.

It would be interesting to test the 21 ZF but it's too pricey for me and considering that I have the 18 ZF and am pleased with it I think I will stay with it.